"If you haven't been keeping up your fitness, when you go to the mountains you'll get tired straight away."
- "If you haven't been keeping up your fitness, when you go to the mountains you'll get tired straight away."
"if you haven't been keeping up your fitness" 「運動不足だったら」"if you haven't been exercising" も使えます。
"when you go to the mountains" 「山に行ったら」
"you'll get tired straight away" 「すぐに疲れる」"you'll tire out straight away." も言えます。